Loop length

Loop lengths combine in the form of course lengths and it is these that influence fabric dimensions and other properties, including weight.Variations in course length between one garment and another can produce size variations, whilst course length variations within structures (particularly when using continuous filament yarns) can produce horizontal barriness and impair the appearance of the fabric.

With the exacting demands of modern knitting technology, the need to maintain a constant loop length at one feed for long periods of time between one feed and another on the same machine, and between different machines knitting the same structure has become of major importance in the control of fabric quality. This requirement has encouraged the development of yarn feed measuring and control devices.

Under normal circumstances, about 15 per cent of the yarn drawn into a newly-formed loop is actually robbed from already-formed neighbouring loops. Although a machine may be set to knit a specific stitch length, fluctuations in yarn or machine variables can affect yarn surface friction or yarn tension and ultimately influence yarn input tension at the knitting point. As a result, the ratio of 'robbed back' to newly-drawn yarn changes and this alters the size of the knitted loop.

Course length measurements can be obtained by unroving the yarn from a knitted fabric. This is time consuming, destructive of material, and only provides information after knitting. Two types of meter may be employed to monitor yarn feed during knitting - yarn length counters and yarn speed meters - which may be considered to be respectively analogous to tachometers and speedometers in cars.

The yarn length counter is simplest in construction, providing a reading of the amount of yarn fed in a certain time period. It is particularly suitable for attaching to a moving yarn feeder on a circular revolving cam-box machine. After a specific number of machine revolutions, the machine is stopped to enable the yarn length reading to be taken; this is then divided by the number of knitting machine revolutions in order to obtain the course length for that feed.

The yarn speed meter may require calibrating and provides a direct reading of the rate of yarn feed, usually in metres per minute, whilst the machine is running. The meter may be hand-held and can be used on a revolving cylinder machine without the need to stop it. To obtain the course length it is necessary to divide the reading by the number of knitting machine revolutions per minute.

Monitoring every feed of a large diameter multi-feeder machine is time-consuming and provides no guarantee that the course length will remain constant after measuring. Positive feed devices are designed to overcome this problem by positively supplying yarn at the correct rate under low yarn tension to the knitting point instead of allowing the latch needles or loop-forming sinkers to draw loops whose length could be affected by varying yarn input tension. HATRA introduced the nip roller positive feed device during the early 1960s. It consists of a lower roller driven by gearing at a speed directly proportional to the machine speed, with an upper, freely running, weighted roller turning in contact with the yarn completing the nip.

Devices of this type tended to have complicated drive linkages, required a complex yarn path, and needed careful adjustment at each device if uniformity of course length was necessary at a number of feeds. For these reasons, the cheaper, simpler, more adaptable, tape positive feed system developed by Isaac Rosen proved to be more acceptable. A continuous tape driven from the machine drive by a single pulley encircles the machine above the feeders and provides identical and constant feed for any yarn threaded through the nip it forms with a free-running feed wheel at each feed position (Figures 13.12 and 22.1). On clockwise revolving machines, the yarn passes from its package into the right-hand side of the tape/wheel nip and on leaving the nip on the left it passes down through a detector to the feeder. The faster the tape speed relative to the machine speed, the faster the rate of yarn feed and the longer the resultant course length. The tape speed is altered by adjusting the scrolled segments of the drive pulley to produce a larger or smaller driving circumference.

Punto di roma, milano rib and double pique require much longer course lengths at the feeders where most needles knit than at the other feeders. For structures of this type, up to four tiers of tapes, each driven at a different speed by a different diameter drive pulley, can be accommodated. Facilities for yarn disengagement are provided and sometimes the yarn is guided around the wheel in a coil to prevent slippage.

Positive Yarn Feeder
Fig. 22.1 Trip-tape positive feed.

Tape positive feed is generally only suitable for structures having a maximum of four different course lengths and requiring a constant course length at each feeder, but some small-area jacquards and diagonal twills can be produced with it. Large-area jacquards and similar structures whose individual needle selection causes large fluctuations in feed rate requirements (both between feeders and at the same feeder from one machine revolution to the next) cannot be supplied from positive-feed devices.

The other type of yarn furnishing device is the storage feeder (Fig. 17.1), which supplies yarn at a uniform tension rather than at a uniform rate of feed and is thus suitable for a wide range of yarn feeds. It may also be used for supplying patterning and weft insertion yarns on some warp knitting machines. Yarn is withdrawn from the package and wound tangentially as equally-spaced coils on a 'store'. Demand at the knitting point causes axial withdrawal of yarn from wraps at the opposite end of the store.

On one design, the spool rotates to wrap the yarn at the top of the store and a lightweight circular plastic comb ensures controlled take-off tension from the base of the store. An inclined disc resting over the wraps senses when they have reached a minimum and switches on the electric motor for the spool drive. It later switches the motor drive off when the required maximum number of wraps have been produced. On another design, the yarn input is through the centre of a stationary spool, with a rotating disc winding the yarn on at the base of the store, the coils being moved upwards by reciprocation of the spool surface. Yarn stop motions and indicator lights are fitted to most units.

A further development is the combination of positive feed and storage feed with a choice of mode available by means of a clutch. With this design, even for positive feed, the tape, which has punched holes, never contacts the yarn; instead it is used to drive a studded wheel and thus wind the yarn onto the store.

Storage feeders provide a store of yarn as the machine stops after a yarn breakage, so it is possible to simplify the yarn path and eliminate the top stop detectors. It is also possible to place yarn packages on supply creels separate from the machine because the storage feeds can compensate for a variation in yarn tension produced by a difference in the angle of the path.

Structures produced with constant and identical course lengths may have a differing or impaired appearance if the allocation of the course length between the knitting elements, and therefore between the components of the stitch structure, varies. Factors that can cause a variation include element timing, element gauge in relation to machine gauge, and the depth of knock-over of one needle bed compared to the other. This effect can be magnified or minimized by the type of structure and yarn, the machine gauge, and the type of relaxation and finishing treatment.

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